1. Field of the Invention
The present invention relates to metal substrates having an ultra-hard composite layer thereon.
2. Discussion of Background Information
Metal surfaces, with the exception of hard metals or specially hardened metals, are generally relatively soft compared to ceramic materials. They are therefore very sensitive toward abrasive media or abrasives. This means that metal surfaces, particularly when they are polished, are very sensitive toward cleaners, steel wool but also other articles which have a tendency to scratch, for example zips or office clips. The metal surfaces then very quickly lose their attractive surface and become matt and unsightly.
However, hardened metal surfaces are also important in other fields. For instance, chromium-hardened steel surfaces are used in machine construction and in the automotive sector in order to harden, for example, pistons or piston rods, cylinder liners and many other surfaces which are subject to wear in order to avoid or reduce wear and thus increase the life. In other processes, the surfaces are, for example, hardened by nitridation or carburization, in which case the diffusion of nitrogen or carbon into the surface produces nitrides or carbides.
The deposition of nitrides or carbides by CVD processes (e.g. TiN, ZrN, vitreous carbon) also enables hard layers to be applied to surfaces. These layers are generally very thin and the associated processes have only limited suitability for large areas and/or complex geometries. In addition, only a very limited number of colors can be produced by CVD processes.
PVD processes are also used for surface layers. In general, these layers are not particularly mechanically and chemically stable due to the usually columnar growth mode.
Ceramic layers can be applied to metal surfaces by flame or plasma spraying processes. These layers are up to several 100 μm thick, are usually very abrasion-resistant, but are usually not transparent and very brittle and usually not resistant to thermal shock.
Thin, transparent layers based on sol-gel systems and nanosize systems can be produced by wet coating processes. DE-A-102004001097 (corresponding to WO-A-2005066388) describes a coating technology by means of which layers having a thickness of only a few μm can be obtained on metal surfaces. Despite this low thickness, the layers are very abrasion-resistant and cannot be scratched by means of, for example, α-alumina-containing scouring sponges. However, they may suffer massive damage when subjected to prolonged action of milling media and scourers based on α-alumina or silicon carbide.
It is an object of the present invention to provide a transparent, translucent or colored coating system for metal surfaces which does not have the above-mentioned deficiencies. It should, in particular, have an extraordinarily high abrasion resistance compared to the known systems and be able to be applied by a wet-chemical coating process. In addition, it should be made possible not only to form an ultrahard protective layer but also give the metal substrate any color.
The object has surprisingly been able to be achieved by a coating composition which comprises precursors for an inorganic, vitreous matrix and fine, highly abrasion-resistant fillers being applied to a metal surface of a substrate and thermally densified, the particle size of the abrasive fillers used being smaller than the thickness of the layer obtained.